There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

First-principles calculations have evolved from mere aids in explaining and supporting experiments to powerful tools for predicting new materials and their properties. In the first part of this review we describe the state-of-the-art computational methodology for calculating the structure and energetics of point defects and impurities in semiconductors. We will pay particular attention to computational aspects which are unique to defects or impurities, such as how to deal with charge states and how to describe and interpret transition levels. In the second part of the review we will illustrate these capabilities with examples for defects and impurities in nitride semiconductors. Point defects have traditionally been considered to play a major role in wide-band-gap semiconductors, and first-principles calculations have been particularly helpful in elucidating the issues. Specifically, calculations have shown that the unintentional n-type conductivity that has often been observed in as-grown GaN cannot be a...

First-principles calculations for defects and impurities: Applications to III-nitrides

Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

/pdf/luminescence-properties-of-defects-in-gan-4hjs241x7f.pdf

Luminescence properties of defects in GaN

Titanium dioxide (TiO2 ) nanocrystals are prepared by a hydrolysis process of tetrabutyl titanate. Nanocrystal samples with various sizes of 6.8-27.9 nm are obtained after annealing from 100 to 650 °C. The crystal structures and the average particle sizes are examined using x-ray diffraction. Raman scattering was employed to investigate the evolution of the anatase phase in the nanocrystals during annealing. Phonon confinement and non-stoichiometry effects are responsible for the blueshift and broadening of the lowest-frequency Eg Raman mode. The influence of interfacial vibrations on the Raman linewidth is also discussed.

https://iopscience.iop.org/article/10.1088/0022-3727/33/8/305/pdf

Raman scattering study on anatase TiO2 nanocrystals

https://hal.archives-ouvertes.fr/hal-00120432/document

Raman Spectroscopy of Nanomaterials: How Spectra Relate to Disorder, Particle Size and Mechanical Properties

Continuous Growth of SiC Single Crystal from Ultrafine Particle Precursor

An experimental investigation was made of spiral instabilities in injected plasmas in a Corbino disk-shaped semiconductor subject to radial electric and axial magnetic fields. Observation of the spiral instabilities was made in p-type germanium at 77°K and in n-type germanium at room temperature. The oscillation frequencies are between 0.5 and 1.6 Mc/s and between 60 and 120 kc/s in p-type and n-type germanium respectively, and depend on the externally applied electric and magnetic fields. It was also observed that there exist some critical ranges for the values of the electric and magnetic fields for which this kind of instabilities occur. A theoretical explanation for most of the experimental results is given.

Spiral Instabilities in Semiconductor Plasmas

Transmittance and phase shift of electromagnetic waves passed through YBa2Cu3O7−δ thin films on MgO substrates are measured in the millimeter wave region (110–170 GHz) below and above Tc to obtain complex dielectric constants. The experimental results are analyzed by the Drude model and two-fluid model above and below Tc, respectively. The temperature dependence of the dielectric constants in the normal phase is in good agreement with that extrapolated from the infrared measurements. The dielectric constant at 60 K indicates that there exists considerable amount of normal carrier even at this temperature.

Dielectric Constants of YBa2Cu3O7−δ Thin Films in the Millimeter Wave Region

The x‐ray diffraction measurement of purple NH4–TCNQ (I) crystal has been carefully carried out near the phase transition temperature (Tc =303 K). The anomalous part of the temperature dependence of the lattice parameter is proportional to the superlattice intensity below Tc. The diffuse scattering can be detected just above Tc.

On the phase transition of purple NH4–TCNQ (I)

Raman spectra of GdBa2Cu3O7-δ thin films (ortho-I, ortho-II and tetra) deposited on MgO substrates by rf magnetron sputtering were measured with an incident light beam whose direction was parallel or perpendicular to the c-axis. The oxygen content was estimated from the frequency of the Cu–O axial stretching mode for these films. The bending mode associated with oxygen at ~320 cm-1 shows the Fano-type asymmetry for all three films indicating the strong electronphonon interaction in this system. This mode softens at around Tc with decreasing temperature for the ortho-I sample whereas it shifts upwards monotonically for the ortho-II and tetra samples. Comparison of this temperature dependence with that of YBa2Cu3O7-δ indicates that the softening of this mode is characteristic for the ortho-I phase.

https://iopscience.iop.org/article/10.1143/JJAP.29.851/pdf

Shin-ichi Nakashima

Papers

Continuous Growth of SiC Single Crystal from Ultrafine Particle Precursor

Spiral Instabilities in Semiconductor Plasmas

Dielectric Constants of YBa2Cu3O7−δ Thin Films in the Millimeter Wave Region

On the phase transition of purple NH4–TCNQ (I)

Characterization of GdBa2Cu3O7-δ thin films by raman scattering